Variable frequency drives traditionally are used in industry to provide variable electric speed to AC motors. These same drives can be used in other applications not related specifically to motors but where a variable-output voltage or frequency is desired. Typical drives have an AC input power source and some type of conversion apparatus, usually using solid-state devices, for converting the fixed AC input voltage into a variable-voltage and/or variable-frequency output. One such type of drive is disclosed in U.S. Pat. No. 5,625,545 to Hammond, which is incorporated herein by reference. The Hammond patent describes a power drive which utilizes a number of series connected power cells arranged and manipulated to produce a three-phase AC output. Connecting multiple power cells in series provides higher voltage output than would be available using only a single cell. Serial connection of cells allows multiple voltage states per phase which are used to obtain improved waveforms. Multiple power cells are provided in each phase output line to a three-phase AC motor. Three-phase AC input power is supplied to each power cell by way of a transformer, which contains multiple three-phase secondary winding circuits, each of which is connected to supply one corresponding power cell with three-phase AC input power. Each power cell controls the power that it supplies to the load using a PWM control method. To reduce harmonics in the source currents, the phase-angle of the secondary winding circuits is shifted, either by zig-zag or extended delta windings, in the Hammond patent.
Each power cell contains an input AC-to-DC rectifier, a smoothing filter, an output DC-to-AC converter, and a control circuit. The input converter accepts three-phase AC input from a secondary winding circuit of the power transformer. The input rectifier, rectifying diodes, transforms three-phase AC power into DC power with significant ripple. To ameliorate the effects of such ripple, a smoothing filter comprised of electrolytic capacitors is connected to the DC side of the input rectifier. The smoothing filter is also connected to the output converter. The output converter is a single-phase H-bridge semiconductor switch utilizing power transistors such as IGBTs. Each transistor of the output converter is operated by a local modulation control circuit. Power for the local modulator control circuit may be obtained within the power cell from the AC power supply.
The Hammond patent also discloses a method of controlling the output of such multiple power cells using a pulse-width modulation technique which selectively controls the duration and frequency of power cell pulse outputs. This method uses control signals, based upon interdigitated carrier signals, to selectively cause a switching event in the output converter of each power cell. The switching events are sequenced such that a switching event is occurring in only one power cell at a time.
One drawback of the power drive described in the Hammond patent is that it provides to only two quadrant operation. However, such medium voltage AC drives are inherently capable of operating in all four quadrants of the speed-torque plane. The first quadrant is where both speed and torque are positive and the third quadrant is where both speed and torque are negative. Thus, in the first and third quadrants, the product of torque and speed is positive. When the product of the torque speed is positive, power flow goes into the mechanical load of the motor. In contrast, second and fourth quadrants are those where the product torque speed is negative, i.e., when the motor is acting as a generator and power is flowing from the mechanical load through the motor and back to the inverter side of the drive. Conventional power drives utilizing multiple power cells, such as the drive disclosed in the Hammond patent, are not presently capable of operating in four quadrants.
It is known in the prior art to use a low voltage drive having a single power cell capable of operating in four-quadrants for providing power regeneration. One example of a four quadrant power cell is disclosed in U.S. Pat. No. 4,788,635 to Heinrich. Heinrich discloses that this is accomplished by controlling the voltage-source inverter on an AC input side to reverse the DC-link polarity thereof, while switching across the DC-link terminal connection to the inverter side thereof, so as to match the polarities. It is disclosed that this is accomplished by the implementation of such matching of polarities with cross-coupled GTO devices associated with the two diodes coupled with the respective poles of a DC-link capacitor between the output of the rectifier thyristor bridge and the DC-link terminal of the voltage-source inverter motor drive. The GTO devices are interrupted with a duty cycle selected in response to the voltage difference between the inverter DC input voltage and a constant reference voltage.
Nevertheless, single power cell configurations as described in Heinrich for providing four quadrant operation can have significant disadvantages due to undesirable harmonics, which are created even at no load situations.
Four quadrant operation can be very desirable in medium voltage AC drive applications because the regenerated power can be used to offset the cost of electricity which must be purchased from utility companies. For some users of such medium voltage, multiple power cell AC drives, energy requirements can be significant, and even a 5-10% power savings, due to the regenerative capability of the drive, can be a significant cost savings. Consequently, it is desirable to provide four quadrant operation for such power drives, without the disadvantage of the undesirable harmonics inherent in the single power cell drives.